Prior to this invention, there have existed no slip joint pliers satisfactory or suitable for use by the typical mechanic such as a garage mechanic for working on motors, or the plumber in use in plumbing activities, for example. In order to be suitable for use in such trades, the pliers must be substantially free of cumbersome parts and construction, and must be easily adjustable. Also there must be security against accidental intermittent release of the mechanism in order to avoid loss of time as well as to avoid potential injury by unexpected release. This is particularly significant in these noted trades where working space is often very small and where accordingly many adjacent objects are apt to be striking the release mechanism of the pliers during their use. Accordingly, the pliers of U.S. Pat. No. 1,003,978 having a side-mounted lever arm for inserting and withdrawing a lock-pin clearly is not a suitable instrument nor tool for adequate use by automobile mechanics and plumbers for reasons stated above. While the mechanism of the pliers of U.S. Pat. No. 1,410,866 is not unduly cumbersome, to change from one to another position requires the prolonged time-consuming procedure of total removal of the small pin and reinsertion of it, removing the catch before removal of the pin and replacing the catch after reinsertion of the pin. Additionally, the hands of mechanics are often soiled and greasy and not most suitable for the handling of a small pin precisely nor carefully nor with ease, with a significantly great possibility that the pin would be dropped and possibly be lost thereby. The pliers of U.S. Pat. No. 1,520,613 having the laterally-movable lever is highly susceptable to the striking of the lever to cause an accidental release, discussed-above. Likewise the locking lever of U.S. Pat. No. 2,811,068 is highly susceptable to being accidentally struck to release the lock mechanism to cause accidental shifting of the pliers jaws relative to their respective pivot points. In addition to the drawbacks noted above, prior slip joint pliers have been of either or both fragile nature and complicated construction.